Optical scanning device

ABSTRACT

An optical scanning device for scanning an information layer of an optical record carrier and including a rotary aim ( 2; 102; 202; 302; 402; 502 ) which is arranged to swing about a rotation axis (CR) to alter an angular position of the rotary arm about the rotation axis; a detector arrangement ( 10 ) arranged separate from the rotary arm ( 2; 102; 202; 302; 402; 502 ) for detecting a radiation beam spot, the radiation beam spot ( 40; 140; 240; 340; 440; 540 ) having an angular disposition; a first reflective surface ( 4; 104; 204; 304; 404; 504 ) attached to the rotary arm ( 2; 102; 202; 302; 402; 502 ); a second reflective surface ( 6; 106; 206; 306; 406; 506 ) attached to the rotary arm ( 2; 102; 302; 402; 502 ); a first light path (LP  1 ; LP  1  O  1 ; LP 201 ; LP 301 ; LP 401 ; LP 501 ; running from a location on the record carrier to said first reflective surface; a second light path (LP 2 ; LP 102 ; LP 202 ; LP 302 ; LP 402 ; LP 502 ) running from said first reflective surface to said second reflective surface; a third light path (LP 3 ; LP 103 ; LP 203 ; LP 303 ; LP 403 ; LP 503 ) running from said second reflective surface to said detector arrangement ( 10 ). The rotary arm includes at least one optical inversion element ( 52; 54; 56; 58; 64; 66 ) arranged such that a dependence between variation of the angular disposition of the radiation beam spot and variation of the angular position of the rotary arm is reduced.

This invention relates to an optical scanning device, particularly to anoptical scanning device comprising a rotary arm for scanning an opticalrecord carrier.

The rotary arm system is known for electromechanical adjustment of aread/write head. A rotary arm scanning mechanism is widely used inmagnetic disc recording/reproducing apparatus, commonly known as harddisc drives, for scanning magnetic discs. The use of a rotary arm hasalso been considered for optical disc recording/reproducing apparatus,for scanning optical or magneto-optical discs. A rotary arm provides asimpler mechanism with a reduced number of parts compared to a 2-stagesledge mechanism, which is the most commonly used scanning mechanism inoptical disc systems.

In known optical scanning devices in which a rotary arm scanningmechanism is used, the optical components, including the laser anddetector system, are located on the moving rotary arm. In such a systemall the control and information signals for the laser and the detectorsystem have to be transferred over a connection foil to and from therotary arm system. In the case of a Small Form Factor Optical (SFFO)device, due to the high speeds and the required noise immunity, even theelectronics for driving the laser and processing the detector signalsmay need to be located on the moving arm. This would result in a thermalproblem with heat dissipation of the laser and its associatedelectronics (driver), a dynamical problem due to the relatively heavyweight of the optical and electrical components, and an interconnectionproblem due to the large amount of electrical connections to the lasercircuitry and the detection circuitry.

These problems can be overcome by using a split optics system. Thisinvolves positioning the laser, the detector array, the electronics andmost of the optics of the optical scanning device at a fixed locationseparated from the rotary arm. In operation of such a system, aradiation beam is reflected by the optical record carrier and directedback along the rotary arm and on to the detector array. Due to avariation of an angular position of the rotary arm about a rotation axisan associated rotation of a radiation beam spot on the detector arrayoccurs. As a result of this rotation an angular disposition of theradiation beam spot falling on the detector array is not consistent.This causes problems for interpretation by the detector array of a datasignal being carried by the radiation beam.

JP 2001-357549 describes an optical scanning device with a rotary arm inwhich the polarization of a radiation beam, having been reflected by anoptical record carrier, is rotated by a rotatable prism which isseparate from the rotary arm. This ensures that an orientation of theradiation beam falling on a detector array is consistent. In order torotate the prism a rotation mechanism is necessary which occupiesadditional space in the optical scanning device and adds complexity toits construction.

In accordance with the present invention there is provided an opticalscanning device for scanning an information layer of an optical recordcarrier, the device including:

a rotary arm which is arranged to swing about a rotation axis to alteran angular position of the rotary arm about the rotation axis;

a detector arrangement arranged separate from the rotary arm fordetecting a radiation beam spot, the radiation beam spot having anangular disposition;

a first reflective surface attached to the rotary arm;

a second reflective surface attached to the rotary arm;

a first light path running from a location on the record carrier to saidfirst reflective surface;

a second light path running from said first reflective surface to saidsecond reflective surface;

a third light path running from said second reflective surface to saiddetector arrangement, characterized in that said rotary arm includes atleast one optical inversion element arranged such that a dependencebetween variation of the angular position of the rotary arm andvariation of the angular disposition of the radiation beam spot isreduced.

The optical inversion element of the rotary arm reduces variation of theangular disposition of the radiation beam spot with a change in theangular position of the rotary arm. By reducing the dependency,techniques such as astigmatic focus error detection may be employed.

Preferably the angular disposition of the radiation beam spot falling onthe detector array is substantially independent of the angular positionof the rotary arm, other than a variation caused by a change in thedirection of data tracks on the optical record carrier across the swingpath of the rotary arm.

It is noted that German patent application DE 198 60054 describes anoptical scanning device for scanning an optical record carrier with arotary arm. This rotary arm has different angular positions about arotation axis and additionally has a first and a second mirror attachedto it. The first mirror receives a radiation beam reflected in adownwards direction from an optical record carrier. This radiation beamis directed along the rotary arm by the first mirror to the secondmirror which reflects the radiation beam, also in a downwards direction,towards a third mirror. In this arrangement an angular disposition of across section of the radiation beam reflected by the second mirror issubstantially independent of the angular position of the rotary arm.However, a build-height in the axial direction of this arrangement isrelatively bulky. It is a further advantage of embodiments of thepresent invention to reduce this build-height such that a more compactoptical scanning device may be constructed.

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, given by way of example only and with reference to theaccompanying drawings.

FIG. 1 shows a plan view of components of an optical scanning device ofa type similar to an embodiment of the present invention;

FIG. 2 shows a side cross section illustrating components of an opticalscanning device of the type similar to an embodiment of the presentinvention;

FIGS. 3 and 4 show schematically cross sections of a radiation beam fordifferent angular positions of a rotary arm of an optical scanningdevice of the type similar to an embodiment of the present invention;

FIG. 5 shows a side cross section of components of an optical scanningdevice in accordance with an embodiment of the present invention;

FIGS. 6 and 7 show schematically cross sections of a radiation beam fordifferent angular positions of a rotary arm in accordance with anembodiment of the present invention;

FIG. 8 shows a side cross section of components of an optical scanningdevice in accordance with an alternative embodiment of the presentinvention;

FIG. 9 shows a side cross section of components of an optical scanningdevice in accordance with an alternative embodiment of the presentinvention;

FIG. 10 shows a side cross section of components of an optical scanningdevice in accordance with an alternative embodiment of the presentinvention;

FIGS. 11 and 12 show schematically cross sections of a radiation beamfor different angular positions of a rotary arm in accordance with anembodiment of the present invention;

FIGS. 13 and 14 show schematically a distortion of a cross section of aradiation beam in accordance with an embodiment of the presentinvention;

FIG. 15 shows a side cross section of components of an optical scanningdevice in accordance with an alternative embodiment of the presentinvention; and

FIGS. 16 and 17 show schematically cross sections of a radiation beamfor different angular positions of a rotary arm in accordance with anembodiment of the present invention.

FIG. 1 shows a plan view and FIG. 2 shows a side cross section ofcomponents of an optical scanning device of a type similar toembodiments of the present invention, but not arranged in accordancewith the invention. The optical scanning device includes a rotary arm 2which is arranged to swing about a rotation axis CR such that an angularposition of the rotary arm 2 can be varied. The rotary arm 2 has abearing system, which consists of two ball elements. One ball element islocated on one part of the rotary arm; the other ball is located onanother part. The ball elements are allowed to rotate in a slightlyoversized bearing shale. The actuation system for the rotary arm 2includes a coil at one end of the arm located in a magnetic field, suchthat one side of the coil is in a vertical upwards field and the otherside of the coil is in a vertical downwards field. A current through thecoil will generate Lorenz forces, which rotate the arm in the desireddirection.

At one end of the rotary arm 2 is attached a first reflective surface,in the form of a first folding mirror 4. A second reflective surface anda third reflective surface, in the form of a second and a third foldingmirror 6, 8 respectively, are stacked along the rotation axis CR. Thesecond folding mirror 6 is attached to the rotary arm 2 and thereforerotates about the rotation axis CR with a rotation of the rotary arm.The third folding mirror 8 is part of a fixed detector arrangement alsoincluding a laser/detector unit 10 which is separate from the rotary arm2. The laser/detector unit 10 includes a radiation source 12, forexample a semiconductor laser, a beam splitter 16, a collimator lens 18and a detector array 20. The radiation source 12 emits a radiation beamwhich is directed by the beam splitter 16 to the collimator lens 18which collimates the radiation beam. This now collimated radiation beamtravels to the third folding mirror 8 which directs the radiation beamalong the rotation axis CR to the second folding mirror 6. The radiationbeam travels from here to the first folding mirror 4 which directs theradiation beam to an objective lens 22 attached to the rotary arm 2. Theobjective lens 22 focuses the radiation beam to a spot 24 at a radiallocation on an information layer 26 of an optical record carrier. Inthis example the optical record carrier is an optical disc OD, forexample a CD, a DVD or a Small Form Factor Optical (SFFO) disc. Theoptical disc OD rotates about a spindle axis SA in order that the spot24 can scan along a track of the information layer 26. In FIG. 1 theoptical disc OD, although not shown directly, is indicated in outline inposition to the above components of the optical scanning device. Theobjective lens 22 and the first folding mirror 4 are mounted on a holderelement (not shown) which is suspended on two parallel flexures. A drivecoil located on the holder element can generate vertical forces suchthat variation of the focus of the radiation beam to the spot 24 can beachieved.

The radiation beam, having been focused to the spot 24 is reflected bythe information layer 26 and travels along a linear first light path LP1to the first folding mirror 4. The first light path LP1 is parallel tothe rotation axis CR. The radiation beam travels from the first foldingmirror 4 to the second folding mirror 6 along a linear second light pathLP2 which is perpendicular the rotation axis CR. The second foldingmirror 6 directs the radiation beam along a third light path LP3 runningfrom the second folding mirror 6 to the detector array 20 of thedetector arrangement 10. A portion of the third light path LP3 betweenthe second and third folding mirrors 6, 8 is linear and coincident withthe rotation axis CR. From the third folding mirror 8 the radiation beamcontinues to travel along the third light path LP3 via the collimatorlens 18, then the beam splitter 16 to the detector array 20. Thedetector array 20 includes detector elements which produce a maininformation signal relating to data stored on the information layer 26of the optical disc OD, a focus error signal and a tracking errorsignal.

Referring now also to FIGS. 3 and 4, cross sections of a radiation beamfor different angular positions of the rotary arm 2 of the opticalscanning device shown by FIGS. 1 and 2, are shown schematically. FIG. 3corresponds to the rotary arm 2 being in a first angular position aboutthe rotation axis CR and FIG. 4 corresponds to the rotary arm 2 being ina second angular position about the rotation axis CR. The second angularposition is displaced about the rotation axis CR from the first angularposition by an angle {tilde over (□)}. The radiation beam travellingfrom the spot 24 to the first folding mirror 4 along the first lightpath LP1 has a first cross section 28. For illustrative purposes thefirst cross section 28 has a first reference axis 30 and a secondreference axis 32 are in the plane of the first cross section 28 andillustrate an angular disposition of the first cross section 28, thesecond reference axis 32 being perpendicular to the first reference axis30. The first light path LP1 is perpendicular to both the first and thesecond reference axes 30, 32 respectively. The first cross section 28has a first and a second region 34, 36 corresponding to the overlap ofthe −1 and +1 diffraction order beams of the radiation beam asdiffracted by the tracks of the optical disc OD, with the first zerothorder main beam. These regions 34, 36 may be used by the detector array20 to perform a radial tracking function, for example one spot push-pullradial tracking, when scanning the information layer 26 of the opticaldisc OD. For further illustrative purposes the first cross section 28 isshown with an off-center reference point 38.

The radiation beam travelling along the third light path LP3 has asecond cross section 40 which corresponds in angular dispositiondirectly with that of the radiation beam spot falling on the detectorarray 20. For similar illustrative purposes the second cross section 40has a first reference axis 42 and a second reference axis 44 which arein the plane of the second cross section 40 and illustrate an angulardisposition of the second cross section 40, and which correspond to thepreviously-described axes 30, 32 in the first cross section 28. Thethird light path LP3 is perpendicular to both the first and the secondreference axes 42, 44 respectively. The second cross section 40 has afirst and a second region 46,48 corresponding to the regions 34, 36 inthe first previously described cross section 28. As described theseregions 46, 48 may be used by the detector array 20 to perform a radialtracking function. For further illustrative purposes the second crosssection 40 has an off-center reference point 50 corresponding to thepreviously-described reference point 38 in the first cross section 28.

With the rotary arm 2 in either the first angular position or the secondangular position, the first cross section 28 has the same angulardisposition. However, with the rotary arm 2 in the second angularposition, as shown in FIG. 4, the second cross section 40 has adifferent angular disposition to the second cross section 40 when therotary arm 2 is in the first angular disposition, as shown in FIG. 3.The angular disposition of the second cross section 40 with the rotaryarm in the first angular position is related to the second cross section40 with the rotary arm in the second angular position by the angle 2α.Additionally, with the rotary arm 2 in the second angular position thesecond cross section 40 is an inverted version of the first crosssection 28 about the second reference axis 32. The different positionsof the off-center reference points 38, 50 of the first and second crosssections respectively illustrate this.

This dependence of the angular disposition of the second cross section40 on the angular position of the rotary arm 2 creates a problem for thepositioning of the detector arrangement 10 of the optical scanningdevice as the angular disposition radiation beam spot falling on thedetector array 20 is not constant, as in known optical scanning deviceswhich use a linear tracking system.

FIG. 5 shows a side cross section of components of an optical scanningdevice in accordance with an embodiment of the present invention.

FIGS. 6 and 7 show schematically cross sections of a radiation beam fordifferent angular positions of a rotary arm in accordance with thisembodiment of the present invention. The radial arm 102 is shownschematically using dotted lines.

Features of this embodiment are similar to those of the optical scanningdevice previously described. Such features are labeled with similarreference numerals incremented by 100 and the descriptions should betaken to apply here also.

In this embodiment of the present invention, an optical inversionelement, which is rigidly fixed to the rotary arm 102, includes a singlereflective surface 57. The optical inversion element comprises a prism,specifically a Dove prism 56. The rotation axis CR is arrangedvertically and the single reflective surface 57 of the Dove prism 56 isarranged horizontally. The radiation beam travelling along the secondlight path LP 102 from the first folding mirror 104 is reflected by thesingle reflective surface 57 of the Dove prism 56 before travelling tothe second folding mirror 106.

Referring now to FIG. 6, with the rotary arm 102 in the first angularposition, the first cross section 128 has the same angular dispositionas the second cross section 140. Referring now to FIG. 7, with therotary arm 102 in the second angular position, the first cross section128 and the second cross section 140 now also both have the same angulardisposition. Additionally the second cross section 140 relative to thefirst cross section 128 is not an inverted version of the first crosssection 128 about the second reference axis 132, as illustrated by thepositions of the respective off-center reference points 138, 150 of thefirst and second cross sections 128, 140.

The angular disposition of the second cross section 140 and the angularposition of the rotary arm 102 are now substantially independent of eachother such that the radiation beam spot falling on the detector array120 is relatively constant.

The angular disposition of the second cross section 140 and the angularposition of the rotary arm 102 are not entirely independent of eachother due to a slight variation of an angular disposition of the firstcross section 128 with respect to the information layer 126 of theoptical disc OD. This results in a variation of the angular dispositionof the radiation beam spot falling on the detector array 120. Theinformation layer 126 comprises tracks which are parallel each otheralong a radius R from the spindle axis SA of the optical disc OD. Inorder for the first cross section 128 to have a constant angulardisposition, the spot 124 would need to scan along the radius of theoptical disc OD from the spindle axis SA. However, when the rotary arm102 is swung about the rotation axis to achieve different angularpositions, the scanning of the spot 124 across the tracks of the opticaldisc OD describes an arc. Therefore the first cross section 128 of theradiation beam has a slight variation in its angular dispositiondependent on the angular position of the rotary arm 102.

In the case where the optical scanning device of this embodiment is aSmall Form Factor Optical (SFFO) scanning device, the optical disc ODhas a radius from the spindle axis SA of between 10 mm and 20 mm, morepreferably approximately 15 mm. Furthermore in this case and in order tominimize the dependency of the angular disposition of the first crosssection 128 on the angular position of the rotary arm 102, it ispreferable that the rotary arm 102 has a length of between 10 mm and 20mm, more preferably approximately 15 mm between the rotation axis CR andthe spot 124. The optical disc OD is preferably arranged such that aninner track R_(in) lies at a radius of between 4 mm and 10 mm, morepreferably approximately 6 mm from the spindle axis SA and that an outertrack R_(out) lies at a radius of between 10 mm and 18 mm, morepreferably approximately 14 mm from the spindle axis SA of the opticaldisc OD. Preferably when the angular position of the rotary arm 102 hasα=0° the spot 124 lies at a radius of approximately central to the innerand outer radii, in the most preferred arrangement approximately 10 mmof the optical disc OD. In the most preferred arrangement, the rotaryarm 102 needs to swing by an angle (α) of approximately −11.5° or +11.5°to scan the inner or outer track accordingly. When the rotary arm 102 isin the position to scan the inner track R_(in) or the outer trackR_(out), where α=−11.5° or α=+11.5° accordingly, then an angulardisposition of the first cross section of the radiation beam 128 isapproximately 0°. When the rotary arm 102 is in a position to scan thetrack approximately central to the inner and outer radii R_(in), R_(out)of the optical disc OD, α=0°. In this position where α=0° the firstcross section of the radiation beam 128 has an angular disposition ofapproximately 3.2°. Consequently a variation of the angular dispositionof the radiation beam spot falling on the detector array 120 of betweenapproximately 0° and 3.2° occurs depending on the angular position ofthe rotary arm 102. Preferably the detector array 120 is arranged suchthat the angular disposition of the radiation beam spot falling on thedetector array 120 varies such that 0° falls approximately in the centerof the range (for example between −1.6° and +1.6°) as this provides anelectrical advantage compared with a variation between for example 0°and 3.2°.

In this arrangement any variation of the angular disposition of thefirst cross section 128 with the angular position of the rotary arm 102and therefore of the radiation beam spot falling on the detector array20 is kept to a minimum.

Minimum and maximum values of the angle of rotation a of the rotary arm102 can be described more generally for an optical disc OD with a radiusR from the spindle axis SA by the following:

$\begin{matrix}{{{Minimum}\mspace{14mu}\alpha} = {- {\arcsin( \frac{x}{L} )}}} & (1) \\{{{Maximum}\mspace{14mu}\alpha} = {+ {\arcsin( \frac{x}{L} )}}} & (2)\end{matrix}$

wherein L is a length of the rotary arm 102 between the rotation axis CRand the spot 124. The rotary arm 102 is preferably arranged such thatapproximately α=0° when a track lying approximately central to the innerand outer radii R_(in), R_(out) is being scanned. During scanning ofthis track, the spot 124 is focused at a position lying along the arcdescribed by the swinging of the rotary arm 102 about the rotation axisCR, not along the radius R; andwherein x=0.5×(R _(out) −R _(in))A distance D between the rotation axis CR and the radius R of theoptical disc OD lies perpendicular the radius R and substantiallyparallel the length L when □=0°. This distance D can be calculated asfollows:D=√{square root over (L²−x²)}.  (3)

FIG. 8 shows a side cross section of components of an optical scanningdevice in accordance with an alternative embodiment of the presentinvention. This embodiment allows a reduced rotary arm mass to beachieved.

Features of this embodiment are similar to those of the optical scanningdevice previously described. Such features are labeled with similarreference numerals incremented by 200 and the descriptions should betaken to apply here also.

As for the previous embodiment an optical inversion element, which isrigidly fixed to the rotary arm 202, comprises a single reflectivesurface which is a mirror 58. The rotation axis CR is arrangedvertically and the mirror 58 is arranged horizontally. In thisembodiment the mirror 58 is attached to an upper surface of the rotaryarm 202. The radiation beam travelling along the second light path LP202from the first folding mirror 204 is reflected by the mirror 58 to thesecond folding mirror 206.

As for the previous embodiment and illustrated using FIGS. 6 and 7, theangular disposition of the second cross section 240 and the angularposition of the rotary arm 202 are substantially independent of eachother such that the radiation beam spot falling on the detector array220 is relatively constant. As described earlier there is also a slightvariation of an angular disposition of the first cross section 228 withrespect to the information layer 226 of the optical disc OD.

In order for the radiation beam travelling along the second light pathLP202 to be reflected by the mirror 58, the first and second foldingmirrors, 204, 206 respectively need to have a specific tilt angle β. Therotary arm 202 has a length L₂₀₂ from the rotation axis CR to the firstlight path LP201. This length L₂₀₂ lies perpendicular the rotation axisCR. The rotary arm 202 also has a height H from the mirror 58 to a pointat which the radiation beam travelling along the second light path LP202strikes the first or the second folding mirror 204, 206. This height His parallel the rotation axis CR. The mirror 58 has a length L₅₈ lyingperpendicular the rotation axis CR which is greater than or equal to theminimum length required for reflection of the radiation beam. Theradiation beam travelling along the second light path LP202 has adiameter d. From this information, the tilt angle β can be calculated,the tilt angle β being the angle of the first or second folding mirror204, 206 with respect to a line perpendicular the rotation axis CR. Thecalculation is as follows:

$\begin{matrix}{\beta = {\frac{1}{2} \cdot {\arctan( \frac{L_{202}}{2H} )}}} & (4)\end{matrix}$

From this the value of the length L₅₈ of the mirror 58 can becalculated:

$\begin{matrix}{L_{58} \geq \frac{d}{\sin( {90 - {2\;\beta}} )}} & (5)\end{matrix}$

For example, if L₂₀₂=15 mm, d=1.5 mm, H=1.6 mm, then β=0.39° and L₅₈≧7.2mm.

FIG. 9 shows a side cross section of components of an optical scanningdevice in accordance with an alternative embodiment of the presentinvention.

Features of this embodiment are similar to those of the optical scanningdevice previously described. Such features are labeled with similarreference numerals incremented by 300 and the descriptions should betaken to apply here also.

In this embodiment the fixed detector arrangement lies on an oppositeside of the second light path LP302 to the optical disc OD. Inpreviously described embodiments, the fixed detector arrangement and theoptical disc OD lie on the same side of the second light path LP 102, LP202.

Referring to FIG. 9, the rotary arm 302 further includes a first opticalinversion element which is a single reflective surface, in this instancea folding mirror 52. The rotary arm 302 includes also a second opticalinversion element which is also a single reflective surface, in thiscase a folding mirror 54. Both folding mirrors 52, 54 are rigidly fixedto the rotary arm 2 and are separated in a direction parallel therotation axis CR. In this embodiment the mirrors 52, 54 are stacked in adirection parallel the rotation axis CR. The radiation beam travellingalong the second light path LP302 from the first folding mirror 304 isdirected to the folding mirror 52 of the optical inversion element, thento the folding mirror 54 of the further optical inversion element andthen further on to the second folding mirror 306.

As for the previous embodiment and illustrated using FIGS. 6 and 7, theangular disposition of the second cross section 340 and the angularposition of the rotary arm 302 are substantially independent of eachother such that the radiation beam spot falling on the detector array320 is relatively constant. There is, however, a slight variation of anangular disposition of the first cross section 328 with respect to theinformation layer 326 as previously described. An advantage of thisarrangement, compared to a similar arrangement in which the twoinverting mirrors 52, 54 are omitted, is that the build height can bereduced.

FIG. 10 shows a side cross section of components of an optical scanningdevice in accordance with an alternative embodiment of the presentinvention. This embodiment allows a reduced number of elements to beused.

FIGS. 11 and 12 show schematically cross sections of a radiation beamfor different angular positions of a rotary arm in accordance with thisembodiment of the present invention. Features of this embodiment aresimilar to those of the optical scanning device previously described.Such features are labeled with similar reference numerals incremented by400 and the descriptions should be taken to apply here also.

Rigidly fixed to the rotary arm 402 is an optical inversion elementwhich is an asymmetric prism 64. The asymmetric prism 64 includes thesecond folding mirror 406 and a single reflective surface 62. Theradiation beam travelling along the second light path LP 402 from thefirst folding mirror 404 enters the asymmetric prism 64 and is thenreflected by the single reflective surface 62 to the second foldingmirror 406.

Referring to FIG. 11, and in a similar manner to previous embodiments ofthe present invention, the angular disposition of the second crosssection 440 is the same as the angular disposition of the first crosssection 428, with the rotary arm 402 being in the first angularposition. In this embodiment however the second cross section 440 isdistorted in the form of a stretch along the first reference axis 442.

Referring now to FIG. 12 in which the rotary arm 402 is in the secondangular position, the angular disposition of the second cross section440 is somewhat dependent on the angular position of the rotary arm 402.The angular disposition about the rotation axis CR of the second crosssection 440 is less than the angle 2α. Similarly for when the rotary arm402 is in the first angular position, the second cross section 440 isdistorted in the form of a stretch along the first reference axis 442.On comparison of the position of the off-center reference points 438,450, of the first and second cross sections respectively, the secondcross section 440 is not an inverted version of the first cross section428.

The distortion of the second cross section 440 of the radiation beam isdescribed more specifically using FIGS. 13 and 14. The radiation beamtravelling along the second light path LP402 has a first diameter d₁before entering the asymmetric prism 64. Following reflection by thesingle reflective surface 62 and the second folding mirror 406, theradiation beam now travelling along the light path LP403 has a seconddiameter d2. The second diameter is equal to a magnification Mmultiplied by the first diameter d₁. This magnification M can becalculated by:M=1/tan θ  (6)

wherein θ is the angle between the single reflective surface 62 and thesecond folding mirror 406. This angle θ is dependent on a refractiveindex of the material from which the asymmetric prism 64 is formed. Theangle θ is preferably selected such that the second light path LP402 andthe portion of the third light path LP403 between the second and thethird folding mirrors 406, 408 lie at an angle of between 80° and 100°,more preferably approximately 90° to each other.

Referring now to FIG. 14, the second cross section 440 has a firstdistortion angle and a second distortion angle, φ₁, φ₂. The firstdistortion angle φ₁ corresponds to a displacement from the firstreference axis 442 with the rotary arm 402 in the first angularposition. The second distortion angle φ₂ corresponds to a displacementfrom the second reference axis 444 with the rotary arm 402 in the secondangular position. The displacement of the rotary arm 402 in the secondangular position from the first angular position is indicated by theangle α. Calculation of the first and the second distortion angles, φ₁,φ₂, respectively is as follows:φ₁ =α−a tan(sin α/M cos α)  (7)φ₂ =a tan(M sin α/cos α)−α.  (8)

FIG. 15 shows a side cross section of components of an optical scanningdevice in accordance with an alternative embodiment of the presentinvention.

FIGS. 16 and 17 show schematically cross sections of a radiation beamfor different angular positions of a rotary arm in accordance with thisembodiment of the present invention.

Features of this embodiment are similar to those of the optical scanningdevice previously described. Such features are labeled with similarreference numerals incremented by 500 and the descriptions should betaken to apply here also.

In this embodiment the optical inversion element comprises a singlereflective surface which is a mirror 66, rigidly fixed to the rotary arm502. The rotation axis CR is arranged vertically and the mirror 66 isalso arranged vertically on an inner surface of a wall of the rotary arm502. The radiation beam travelling along the second light path LP502from the first folding mirror 504 is reflected by the vertical mirror 66to the second folding mirror 506. A radiation beam contact spot 68 isillustrated showing the position of reflection of the radiation beam onthe mirror 66.

Referring now to FIG. 16 where the rotary arm 502 is in the firstangular position, the second cross section 540 has an angulardisposition which is different to the first cross section 528.Additionally reflection of the radiation beam by the vertical mirror 66causes the second cross section 540 to be an inverted version about thefirst reference axis 542 of the first cross section 528. This isillustrated by comparison of the off-center reference points 538, 550 ofthe first and second cross sections respectively.

Referring now to FIG. 17 where the rotary arm 502 is in the secondangular position, the second cross section 540 again has a differentangular disposition to that of the first cross section 528. This angulardisposition of the second cross section 540 when the rotary arm 502 isin the second angular position is the same as the angular disposition ofthe second cross section 540 when the rotary arm 502 is in the firstangular position. Additionally, the second cross section 540 is the sameinverted version of the first cross section 528.

Therefore, in a similar manner to previous embodiments of the presentinvention the angular disposition of the second cross section 540 andthe angular position of the rotary arm 502 are substantially independentof each other such that the radiation beam spot falling on the detectorarray 520 is relatively constant.

The above embodiments are understood to be illustrative examples of theinvention. Further embodiments of the invention are envisaged.

In embodiments of the present invention the optical inversion element isrigidly fixed to the rotary arm. Alternatively the optical inversionelement may be attached to the rotary arm in such a manner that, forexample, rotation or translation of the element on the arm can beachieved.

Furthermore it is envisaged that the optical inversion element is notlimited to being positioned on the rotary arm along the second lightpath. Alternatively the element may be located along the third lightpath.

In the above embodiments the described radial tracking system is of aone-spot push-pull type. It is envisaged further that of a three-spotpush-pull type may alternatively be used.

In the above embodiments mirrors and prisms are used as opticalinversion elements. Other types of element may alternatively be used.

It is to be understood that any feature described in relation to any oneembodiment may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the embodiments, or any combination of any other of theembodiments. Furthermore, equivalents and modifications not describedabove may also be employed without departing from the scope of theinvention, which is defined in the accompanying claims.

1. An optical scanning device for scanning an information layer of anoptical record carrier, the device including: a rotary arm (2; 102; 202;302; 402; 502) which is arranged to swing about a rotation axis (CR) toalter an angular position of the rotary arm about the rotation axis; adetector arrangement (10) arranged separate from the rotary arm (2; 102;202; 302; 402; 502) for detecting a radiation beam spot, the radiationbeam spot (40; 140; 240; 340; 440; 540) having an angular disposition; afirst reflective surface (4; 104; 204; 304; 404; 504) attached to therotary arm (2; 102; 202; 302; 402; 502); a second reflective surface (6;106; 206; 306; 406; 506) attached to the rotary arm (2; 102; 302; 402;502); a first light path (LP1; LP101; LP201; LP301; LP401; LP501)running from a location on the record carrier to said first reflectivesurface; a second light path (LP2; LP202; LP302; LP402; LP502) runningfrom said first reflective surface to said second reflective surface; athird light path (LP3; LP103; LP203; LP303; LP403; LP503) running fromsaid second reflective surface to said detector arrangement (10),characterized in that said rotary arm includes at least one opticalinversion element (52; 54; 56; 58; 64; 66) arranged such that adependence between variation of the angular position of the rotary armand variation of the angular disposition of the radiation beam spot isreduced.
 2. An optical scanning device according to claim 1, wherein theangular disposition of the radiation beam spot and the angular positionof the rotary arm (2; 102; 202; 302; 402; 502) are substantiallyindependent, after taking into account a variation caused by a change inthe direction of data tracks on the optical record carrier across theswing path of the rotary arm.
 3. An optical scanning device according toany of claims 1, wherein said radiation beam spot comprises regions (46;48; 146; 148; 446; 448; 546; 548) corresponding to first diffractionorders of said radiation beam for use in a radial tracking function. 4.An optical scanning device according to claim 1, wherein a portion ofthe third light path (LP3; LP103; LP203; LP303; LP403; LP503) issubstantially coincident with the rotation axis (CR).
 5. An opticalscanning device according to claim 1, wherein the optical inversionelement is rigidly fixed to the rotary arm.
 6. An optical scanningdevice according to claim 1, wherein the optical inversion elementincludes only a single reflective surface (52).
 7. An optical scanningdevice according claim 1, wherein the optical scanning device comprisesa further optical inversion element (54), wherein the optical inversionelement and said further optical inversion element are separated in adirection parallel to the rotation axis (CR).
 8. An optical scanningdevice according to claim 1, wherein the optical inversion elementcomprises a prism.
 9. An optical scanning device according to claim 8,wherein the prism includes the second reflective surface (406).
 10. Anoptical scanning device according to claim 9, wherein the prism is anasymmetric prism.
 11. An optical scanning device according to claim 1,wherein the optical inversion element comprises a mirror (52; 56; 58;66).